This is the boss. No calculator — the atomic masses are posted whole, the sample masses are chosen to divide cleanly, and Avogadro rides at $6.02 \times 10^{23}$. One sample, one long problem, and every answer feeds the next: build the molar mass from the atoms, weigh out the moles, hand them to Avogadro, then say what share of the mass one element carries.

Then one last jar, with somebody else's handwriting on it — and this time the question is not a number but a verdict. Write each line down as you go; one early slip rolls all the way to the signature. 5★ under par here is the chapter crown. Clipboard up.